Various 4-(5-halomethyl-2-pyridyloxy)phenoxy compounds are known to be useful as herbicides as disclosed in European Published Patent Application No. 483, United Kingdom Patent Specification Nos. 1,599,121 and 1,599,126 and U.S. Pat. Nos. 4,184,041 and 4,317,913. For example, butyl 2-[4-(5-trifluoromethyl-2-pyridyloxy)phenoxy]propionate which is also known as fluazifopbutyl is an effective grass herbicide which can be used in fields where broad-leaved crops such as cotton and soybeans are cultivated. Important starting materials for such pyridyloxyphenoxy compounds are the 2-halo-5-trichloromethylpyridines such as 2-chloro-5-trichloromethylpyridine described in U.S Pat. No. 4,317,913. Such 2-halo-5-trichloromethylpyridines, in turn, may be prepared by chlorinating, under ultraviolet light irradiation, a 2-halo-5-methylpyridine as described in U.S. Pat. No. 4,152,328. One starting material for 2-halo-5-methylpyridine is the corresponding hydroxy compound 2-hydroxy-5-methylpyridine, also known as 5-methyl-2-pyridone, of the following formula (I): ##STR1## as disclosed by W. Herz et al. in the Journal of Organic Chemistry, Vol. 26 pages 122-125 (1961).
In the proposed synthetic schemes for the pyridyloxyphenoxy herbicides one object has been the synthesis of 2-halo-5-methylpyridines without utilizing pyridine, and in particular 3-picoline, picoline, starting materials to thus avoid the problems of by-product formation in the halogenation reaction to yield 2-halo-5-methylpyridine. This object was achieved in copending U.S. Ser. No. 433,273 filed Oct. 7, 1982, now U.S. Pat. No. 4,473,696, assigned to the assignee of the present invention. In U.S. Pat. No. 4,473,696, the pyridone of formula (I) was synthesized from propionaldehyde and an acrylic compound of the formula CH.sub.2 .dbd.CH-Y, where Y.dbd.COOR, --CONH.sub.2 or CN and R is an organic moiety such as alkyl, through 5-methyl-3,4-dihydro-2(1H)-pyridone of the following formula (II): ##STR2## The dihydropyridone of formula (II) was disclosed as being dehydrogenated, or oxidized, to (I) by a two-step process comprising the halogenation of (II) across the double bond followed by dehydrohalogenation with loss of two moles of hydrogen halide to thus produce two double bonds which results in the structure (I).
Dehydrogenation of various dihydro hydrocarbon and hetroatomic rings is known to proceed with sodium nitrite and sulfuric acid, palladium on carbon, sulphur and selenium, usually at very high temperatures, e.g., over 200.degree. C. Literature citations include C. Seoane et al. in Heterocycles, Vol. 14, No. 5, pages 639-642 (1980); F. Al-Hajjar et al. in the Journal of Heterocyclic Chemistry, Vol. 17, pages 1521-1525 (1980): M. Trolliet et al. in the Bulletin de la Societe Chimique De France, No. 7-8, pages 1484-1486 (1974); T. Shono et al. in the Journal of Organic Chemistry, Vol. 46 page 3719 (1981): M. Shamma et al. in the Journal of Organic Chemistry, Vol. 26, pages 2586-2587 (1961); L. Davies et al. in the Journal of the Chemical Society, Perkins Transactions I, pages 1293-1297 (1978): E. Spaeth et al. in Berichte der Deutschen Chemischen Gesellschaft Vol. 69, pages 2060-2061 (1936): in "Methoden der Organischen Chemie" (Houben Weyl) Band IV/A, Georg Thieme Verlag, Stuttgart, pages 391-418 (1981) and particularly pages 405-416: and in "Reagents for Organic Synthesis" by L. F. Fieser at pages 1118-1120, John Wiley and Sons Inc. (1967). However, disadvantages of prior processes include the problem of containing the highly poisonous H.sub.2 Se when using Se.degree. as the dehydrogenation agent, the use of high temperatures which increases production costs and which may cause excessive by-product formation, the use of catalysts which cause disproportionation, e.g., the production of both aromatic and saturated species from an unsaturated starting material, and the incorporation of one or more atoms of the reagent used into the molecule to be dehydrogenated, e.g., exchange of oxygen in the starting material with sulphur in the dehydrogenation agent.
It is an object of the present invention to provide a one step, low temperature, relatively mild reaction to synthesize (I) from (II) in high yield and with an inexpensive and safe reagent as well as under safe reaction conditions.